Late Cretaceous strata of the Western Interior Basin, USA, preserve astronomically influenced sedimentation and abundant volcanic ash beds, providing a remarkable opportunity to develop integrated astronomical and radioisotopic time scales. Chao Ma and colleagues used X-ray fluorescence (XRF) core scanning to develop a new elemental data set for cyclostratigraphic investigation of Cenomanian/Turonian strata, including the uppermost Lincoln Limestone Member, the Hartland Shale Member, and the Bridge Creek Limestone Member. 40Ar/39Ar ages from ashes in three biozones, including a new age from the uppermost Lincoln Limestone Member, provide geochronologic constraints for the cyclostratigraphic analysis. Ma and colleagues note that results from the Bridge Creek Limestone Member are consistent with the previously published astrochronology from the USGS #1 Portland core. They note that identification of an astronomical signal in the underlying Hartland Shale Member permits extension of the Western Interior Basin astrochronology into the earlier Cenomanian, prior to Oceanic Anoxic Event 2 (OAE2). High rates of sedimentation in the Angus core during the interval of OAE 2 initiation allow recognition of a strong precessional control on bedding development. As a consequence, the new results provide a rare high-resolution chronometer for the onset of OAE 2, and the timing of proposed hydrothermal trace metal enrichment as observed in the XRF data.
Landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 Ma, Atacama Desert, Chile
Teresa E. Jordan et al., Department of Earth & Atmospheric Sciences, Cornell University, Ithaca, New York 14853-1504, USA. Published online 1 Apr.
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Geological Society of America